Actually, I think that's a great answer! I always remind people about the "evolution of design" - and how different guns developed for different purposes... and that's why there's different types. And shooting a shotgun, for instance, is much different than shooting a pistol. Then, if you factor in the "hobbyist" aspect and, for some, the "collector" aspect … well, it's not at all surprising to see an avid shooter with a "collection" of guns.
Then I say this: it's really no different than walking into the garage or laundry room of an avid skier and seeing a couple pair of downhill skis, a set of cross-country skis, perhaps a pair of slalom skis, a snowboard or two... and then walking into their sitting room and seeing several pair of antique skis displayed on the wall. Same exact thing! Hobbies tend to lead to collections. And you can extrapolate that to ANY hobby really - a photographer, a golfer, a car enthusiast, etc. Do you NEED more than one camera? Do you NEED more than one golf club? Do you NEED more than one car? Of course not, but if you're a hobbyist, you'll enjoy having several. If you explain that really nicely and casually, the light bulb usually goes off.
And yeah, as others have said, get this guy to the range to try your AR-15!